In wireless microwave links, radios are equipped with microwave filters, a typical example of such a filter is a diplexer between antenna, transmitter and receiver. These filters have to be adjusted to different frequencies within a band; this is today made in the assembly factory or by subcontractors. It would be desirable to allow a customer to set the frequency after the equipment is installed, and also to have the opportunity to change the frequency during the life of the product.
Electrical tuning of waveguide E-plane filters for radio communication is a well-known technology. By way of example, pin diodes, varactor diodes or ferroelectrics can be used to set the correct frequency. The usual way to implement electrical tuning devices in a waveguide is to vary the capacitive and inductive coupling in the waveguide and also to design additional structures in the waveguide to concentrate the electric field to the position where the tuning device is placed such as insertion of dielectric materials.
However, tuning of waveguide filters and other waveguide structures using pin diodes, ferroelectrics etc., will affect the general electrical performance of the waveguide filter in a negative way. One of the absolute major problems is the resulting low effective Q-factor when for example using pin diodes or ferroelectric structures in a waveguide filter, which in turn results in high losses.
One way to obtain tuning of waveguide E-plane filters is described in WO 2012/016584, where at least one row of switches is used for creating a virtual wall that is possible to turn on and turn off. Such switches may for example be realized by means of MEMS (Micro Electro Mechanical Systems). Such an arrangement provides a far better Q-factor than previous arrangements, but there is still a desire to obtain a less expensive tuneable waveguide E-plane filter with even better Q-factor.
There is thus a need for an enhanced waveguide tuning device that is as well as an enhanced electrically tuneable waveguide structure, where the enhancement manly relates to the Q-factor and manufacturing costs.